1. Field of the Invention
The present invention relates to a process for producing esters by the acid-catalyzed azeotropic esterification of a hydroxy component and an ethylenically unsaturated carboxylic acid followed by the reaction of any residual acid with an ethylenically unsaturated mono-epoxy compound.
2. Description of the Prior Art
Esters of (meth)acrylic acid are employed, as reactive thinners, for example, in coating technology as well as in sealant compositions. These esters are (meth)acrylic acid esters which are based on mono- or polyhydric alcohols. The (meth)acrylic acid esters are generally produced by the azeotropic esterification of (meth)acrylic acid with mono- or polyhydric alcohols in an inert solvent using acidic catalysts, in the presence of stabilizers and while passing air through the reaction mixture. After esterification, the inert solvent is removed by distillation. The resulting reactive thinners are used in combination with unsaturated lacquer resins, and occasionally also in combination with saturated lacquer resins, to obtain viscosities which are suitable for application. Curing of the resulting compositions can be effected by peroxide accelerator systems (conventional curing), by UV radiation using photoinitiators, or directly by using an electron beam.
These esters are also used in the form of polyether acrylates. This class of compounds is produced analogously to the reactive thinners, by the azeotropic esterification of (meth) acrylic acid with polyether alcohols of higher functionality. Their area of application is mainly as coating compositions curable by UV radiation or electron beam. Polyether acrylates can be combined with unsaturated lacquer resins, reactive thinners or inert solvents.
A further area of application is their use in the form of polyester acrylate resins. For the production of these so-called polyester acrylate resins, pre-condensates (polyester polyols) are produced from polyhydric alcohols and mono- or dibasic, saturated or aromatically unsaturated carboxylic acids by condensation in the melt. These pre-condensates must contain at least two hydroxyl groups per molecule, and are then reacted, analogously to the formation of the reactive thinners or polyether acrylates, by the acid-catalyzed azeotropic esterification with (meth)acrylic acid. Polyester acrylates are also predominantly cured by high-energy radiation and can be employed in combination with inert solvents or reactive thinners.
The (meth)acrylic acid esters can also be used as precursors for the production of other unsaturated resins, such as urethane acrylates or amine-modified polyether acrylates, for example.
During the production of (meth)acrylic acid esters by the previously discussed known process, a residual acid number of up to 20 (mg KOH/g resin) remains in most cases. However, this residual acid number imparts corrosive properties to the products, which can result in the formation of rust in containers, for example, which leads to contamination of the product and the de-stabilization of the product by metal ions. Residues of (meth)acrylic acid are also discernible by their odor and occasionally because they irritate the skin during handling operations. Accordingly, there have been many attempts to remove residual acids from the resins produced.
During the production of (meth)acrylic acid esters, the batch is washed with water or optionally with dilute caustic soda after azeotropic esterification to remove acid catalysts and possibly other acids. A residual acid number of up to 5 (mg KOH/g resin) remains in most cases, however. Also, emulsions which are difficult to separate can be formed.
Attempts have also been made to solve this problem by adding dissolved lime after the production of the (meth)acrylic acid ester, and removing the resulting insoluble calcium salt by filtration (e.g. U.S. Pat. No. 3,717,672). However, oily precipitates, which can only be filtered with difficulty, are often formed.
In addition, methods of producing (meth)acrylic acid esters are known in which after neutralization of the esterification catalyst the residual (meth)acrylic acid is reacted with an epoxy compound (e.g. EP-A 127 766, EP-A 54 105). Depending upon the type and amount of epoxide used, the viscosity of the final product also changes to a varying extent. Also, the neutralization products cannot be polymerized, i.e. they remain as constituents which cannot be chemically incorporated into the resulting polymer. This results in unwanted film properties during subsequent curing, e.g., low hardness or extractability.
It is an object of the present invention to produce (meth)acrylic acid esters such that the final product has a reduced acid number, without disadvantageously affecting the properties of polymerizable mixtures produced therewith, such as color, viscosity or reactivity, and without disadvantageously affecting the properties of lacquer films produced therefrom, such as hardness or extractability.
This object has been achieved in accordance with the present invention by reacting the acid catalyst and the residual (meth)acrylic acid with an ethylenically unsaturated monoepoxy compound after the production of the desired (meth)acrylic acid ester. This results in a significant decrease in the acid number to a value of less than 1 mg KOH/g resin. In addition, the resulting binder composition exclusively contains reaction products which are polymerizable and, thus, can be chemically incorporated into the final product.